Automotive transmissions often have hydraulically-actuated, selectively engagable torque-transmitting mechanisms, referred to as clutches. The clutches may be stationary-type clutches (i.e., brakes), or rotating-type clutches. Typically, interspaced sets of friction plates and reaction plates are placed in contact with one another when the clutch is engaged, to cause common rotation (in the case of the rotating-type clutch) or grounding (in the case of the stationary-type clutch) of components connected with the respective sets of plates.
Hydraulically-actuated clutches have inherent spin losses which reduce fuel economy. Spin losses are caused by fluid in the clutch increasing resistance to rotation of the components connected with the clutch plates. Additionally, such hydraulically-actuated clutches are not engagable until sufficient fluid pressure is available from a transmission pump. Typically, a main transmission pump is driven by an engine that is connected with the transmission or is driven by a rotatable transmission member, and so is indirectly driven by the engine.
When the engine is rotating at relatively low speeds, or if the engine is completely off, as may be the case with an engine having an automatic start/stop capability, the main pump may not supply sufficient hydraulic pressure. Accordingly, auxiliary pumps, such as electrically-powered pumps, are sometimes employed to provide the requisite hydraulic pressure when pressure from the main pump is insufficient.